Khedive's Star — awarded by the Khedive of Egypt to the British troops who installed him in power during the 1882 Anglo-Egyptian War. 2nd Royal Dublin Fusiliers, Mr Mack's battalion, only arrived in Egypt in 1885 – some time after the main fighting had finished. The Khedive's Star, however, continued to be issued until 1891. ((British Army Medals.))

brummagem — cheap, showy trinket. (("counterfeit, sham, not genuine; of the nature of a cheap or showy imitation" – OED; from "local vulgar form" of "Birmingham".))

there's a war on ... from the catchphrase "don't you know there's a war on?" (("frequently employed with jocular irony, e.g. for 'hurry up!' or as a palliative, 'After all, there is a war on'" WW1, revived WW2 – Partridge, Dictionary of Catchphrases.))

aetatis a nipper — misuse of Latin aetatis "of the age of" which expects a specific age to follow. ((nipper: a boy "especially if under 12" – Partridge.))

battle honours — (("distinctions commemorative of battles which are placed, by order of government, on a regiment's colours" – Oxford Companion to Military History.)) Arcot, Condore ... — from the particular honours of the Royal Dublin Fusiliers.

the Stilton — even better than "the cheese" (Stilton being "the king of cheeses").

heaven be her bed tonight — pious expression with the effect of "may she rest in peace".

Acacia caffra

mimosa — apparently Acacia caffra, family Mimosoideae. An Afrikaans name for this is "Wag'n Bietjie" which translates as "wait a bit" – from the habit of the thorns pulling one back. ((Trees of Southern Africa))

upped his age — pretended he was older than he was upon enlistment. (The lower age limit for enlistment at the time was 18 years. But the recruit had to be aged 19 to serve abroad. It seems likely it was the latter age that Gordie "upped" his to.) ((1914-18.net))

thick — (("closely acquainted" – PWJ 1910.))

the world and his wife — everybody. (("a very large number of people" – Cambridge Idioms.))

A credit to the nation ... — regimental song, save the Irish Times, May 1 1915, calls it a "street song". In the 1960s The Dubliners recorded an updated version. ♫♫Irish Times report

good-conduct badge — an error. Soldiers were not even eligible for a good-conduct badge (chevron on the lower sleeve) until after 2 years of service. ((Great War Forum)) skill-at-arms badge — badge worn on lower sleeve for proficiency in particular weaponry.

Come Back to Erin — sentimental ballad, often played at departures in Ireland, though it was in fact written by an English lady – "Claribel" (Charlotte Alington) in 1866. ♫♫

has the dustman passed? — ((dustman: "personification of sleep or sleepiness; in allusion to the rubbing of the eyes as if there were dust in them" – OED.))

club-books — the account books for the various savings clubs Mr Mack organizes. tally fortnights — his fortnightly collections. ((tally: "the record of an amount due; an account" – OED.)) but cf ((tallyman: "one who supplies goods on credit, to be paid for by instalments" – OED.))